The Galindo Cartel are first seen in season four, their primary source of income is cocaine trafficking. They are based in the Mexican state of Sonora and have an ongoing conflict with the Lobos Sonora Cartel.
They are first seen in episode four, first introduced to the Sons by the Mayan leader Marcus Alvarez, where the cartel leader and his associates are shown a heavy arsenal of weapons hidden in a crashed car and are then tested by Romeo's associates. Romeo agrees to regularly buy guns from the Sons, and then tells Clay to pick up the cocaine soon.
It is at this point where Jax finds out that Clay has secretly agreed to mule cocaine for the cartel; a risky move that no one in the club had known about. Clay claims he had no choice, saying that the Galindos were the only force powerful enough to stop the Russians from retaliating against the Sons after the events of Opie's wedding. Jax immediately sees through Clay's own greed, where he then admits that he needs money for his retirement as there has been little improvement from the club's efforts in the last few years.